The relative motion and interaction between the Antarctic, Australia
and Pacific tectonic plates have been poorly understood and have fueled
disagreement over misfits in the global tectonic circuit. The region between
Marie Byrd Land of Antarctica; Tasmania, Australia; and New Zealand was
believed by scientists to have potentially held the final missing piece
of the Antarctic tectonic puzzle.

Steven Cande of the Scripps Institution of Oceanography led a team of
researchers on two extended research cruises into the Northern Ross Sea
and South Tasman Sea. They collected copious bathymetric, magnetic anomaly
and fracture zone data and published their findings in the March 9 issue
of Nature. “The East-West Antarctica problem was the last missing
link where we thought we could go out, collect data and actually solve
the problem,” Cande says.

Bathymetric data have revealed part of the missing link — a trough running
north-northwest out of the Ross Sea Embayment of Antarctica. Dubbed the
Adare Trough, it is a graben structure, or a basin-like structure evident
of crustal extension.

Paleomagnetic data provide evidence that it was also the site of an
ancient spreading center that formed a triple-ridge junction between the
Australia, Antarctic and Pacific plates during the Eocene and Oligocene.

“The biggest uncertainty remaining for the Cenozoic global circuit is
the uncertainty of deformation within the Antarctic Plate,” says Tanya
Atwater of the University of California, Santa Barbara. “Every development
in pinning that down is a huge boon for all studies of the Circum-Pacific
rim.”

At a mid-ocean spreading center, tectonic plates are pushed apart as
new crust is extruded and added onto the plate. As newly formed continental
crust cools, iron-rich minerals record the polarity of the geomagnetic
field. The history of geomagnetic reversals is recorded at spreading centers,
providing information about the duration and time of spreading.

Paleomagnetic data collected by Cande’s team revealed geomagnetic reversals
in the sea floor that run parallel to the Adare Trough and recorded spreading
along an ancient ridge that took place during the Eocene and Oligocene.
The majority of spreading took place between 43 and 26 million years ago
at an average rate of 12 millimeters per year, a rate comparable to the
slowest rates of seafloor spreading observed today.

Cande’s team says spreading in the Adare Trough probably accounts for
over 180-kilometers of extension and 40 to 90 degrees of relative rotation
between East and West Antarctica. This corresponds to the opening of the
Ross Sea Embayment, now overlain by the Ross Sea Ice Shelf. The paleomagnetic
data are supported by the presence of Cenozoic sediments found in drill
cores retrieved in 1999 near Cape Roberts, Antarctica.

There are many global geologic features associated with Pacific Plate
motion that scientists have not been able to account for without reasonably
confident motion constraints. The root of this problem has been the lack
of evidence for sea-floor spreading centers along the Pacific Plate boundary.

“For many decades there has been a disconnect between absolute motion
results using fixed hot spots under the Hawaiian-Emperor Chain and relative
results using seafloor data,” Atwater says. “Any improvement in our understanding
of how hot spots move will make a significant contribution to helping us
understand global tectonics.”

By constraining motion in Antarctica, Cande’s team hoped, a correlation
between volcanic arc movements around the globe could be made to determine
fixed-mantle hot spots on Earth. They remodeled the formation of the 120-degree
bend in the Emperor-Hawaiian Chain in light of the new Antarctic data.
Cande and others did not observe any correlation with movement in other
volcanic arcs in the Indian Ocean. They now believe that hot-spot migration,
not plate movement over fixed mantle sources, is the more probable answer
to the volcanic arc migration enigma.

Regionally, the reconstruction of New Zealand through time has long
been a mystery for scientists who have sought to account for the 150-kilometer
gap along the transform plate boundary that forms the Alpine Fault through
the South Island of New Zealand. Accurate constraints on extension and
rotation between East and West Antarctica have allowed scientists to reconstruct
models that account for the Adare Trough spreading and to close the gap
between the Lord Howe Rise and the Campbell Plateau, fixing the New Zealand
misfit.

The Adare Trough discovery and associated motion between East and West
Antarctica has also posed some new questions related to the uplift of the
Transantarctic Mountains that flank the Ross Sea. Cande believes that spreading
along the Adare Trough may have commenced at a very slow rate before 46
million years ago, coinciding with early uplift of the Transantarctic Mountains
55 to 50 million years ago. He and others are currently working on a proposal
to continue research in the region that could potentially include an investigation
of the relationship between ridge spreading and Transantarctic uplift.